The invention relates to a method and an apparatus for detecting fluid influx from an earth formation to a wellbore or fluid efflux from the wellbore to the formation. Fluid efflux is often referred to as lost circulation. A sudden influx of gas, water or oil during drilling of the wellbore, or during any other operation in the wellbore, is referred to as a kick.
Gas which has entered the wellbore flows upwards and expands during the upward flow because of the decreasing hydrostatic pressure. The expanding gas displaces an increasing volume of liquid in the wellbore. If the kick is not detected at an early stage, the accumulated gas in the wellbore may displace a significant part of the liquid column in the wellbore, which may lead to a blowout if control of fluid flow in the wellbore is lost.
Early detection of a kick is of utmost importance to enable appropriate measures to be taken quickly so that control of fluid flow in the wellbore is maintained. Usually the wellbore is shut in using a blowout preventer, and a heavier fluid is pumped into the wellbore so as to increase the hydrostatis pressure in the wellbore.
The importance of early detection of a kick is even more pronounced when the wellbore has a relatively small diameter since the volume of the liquid column in the wellbore is then relatively small, and therefore the fluid column will be displaced quicker if gas has entered the wellbore. Drilling of wellbores with a relatively small diameter is referred to as slimhole drilling.
A conventional technique for detecting fluid influx or efflux during offshore drilling consists of monitoring the total volume of drilling fluid in the mud pit which is a reservoir for drilling fluid returning from the well. Furthermore, the flow-out of drilling fluid can be monitored in an outlet pipe of the drilling riser, which outlet pipe is referred to as the bell nipple. When the well is being drilled from a floating drilling platform this technique is inaccurate because of movement of the floating drilling platform or floating structure, which movements cause variations of the fluid level in the mud pit and variations of the flowrate in the bell nipple.
Conventional techniques for detecting kicks are in many cases too slow to be used slimhole drilling. Furthermore, these conventional techniques are often inaccurate.
It is an object of the invention to overcome these and other drawbacks of known methods and systems for detecting outflow of fluid from a wellbore or inflow of fluid to the wellbore.